ABSTRACT
Current transformers (c.t’s) step down the primary system currents of up to a few thousand amperes to a level which the measurement and protection circuits can handle, usually 1 or 5 A at rated primary current. In the case of simple overcurrent protection devices, the pick-up current of the protection referred to the primary is used as reference current. The main disadvantages of linearized c.t’s are the imprecise transformation of the DC component in the fault current, which is inadmissible for certain kinds of protection, and the slow decay of the secondary current after the primary current has been interrupted. Capacitive voltage transformer’s are also used as well as inductive c.t’s at power system voltages higher than HOkV, because they are cheaper for the higher voltages and have the added advantage of also being able to use the capacitive voltage divider needed for measurement as a coupling capacitor for power line carrier communication.
